1. Field of the Invention
The present invention relates to a diplexer that can find applications in mobile communication apparatus adapted to operate with microwaves and also to a multiplexer using the same. More particularly, the present invention relates to a diplexer and a multiplexer that have a high-frequency side pass band at or near 5 GHz and are required to secure attenuation in a frequency domain not lower than 10 GHz as in the case of WiFi. WiFi (Wireless Fidelity) is a technology brand invented to improve the consumers' recognition of the “IEEE802.11a/IEEE802.11b” standards for wireless LANs. IEEE802.11a is one of the standards established for LAN technologies by the 802 Committee of IEEE that provides specifications for wireless communications at about 54 Mbps in the 5.2 GHz band. In Japan, 5.15 GHz to 5.25 GHz are assigned to high speed wireless LANs. IEEE802.11b is a standard providing specifications for wireless communications at about 11 Mbps in the 2.4 GHz band.
2. Description of the Related Art
There are known separators prepared for band separation in dual band portable phones that can operate for communications by way of two frequency bands and by means of a single potable terminal by integrally laying ceramic insulating layers to produce a laminate and forming therein a first notch circuit having a parallel circuit of an inductance and a capacitance and a second notch circuit also having a parallel circuit of an inductance and a capacitance, the line width of the inductance of the high-frequency side second notch circuit being made grater than the line width of the inductance of the first notch circuit, for the purpose of downsizing, while maintaining the characteristics of the separator (see Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 11-68499).
Similar separators that can be easily downsized with a small loss and includes a low-frequency side first filter, which is a low pass filter equipped with an attenuation pole, and a high-frequency side second filter, which is a high pass filter also equipped with an attenuation pole, one of the input/output ports of the first and second filters being made to operate as common port, are also known (see Patent Document 2: Jpn. Pat. Appln. Laid-Open Publication No. 11-266133).
Furthermore, separators including a first filter and a second filter having respective center frequencies that are different from each other, and a separation circuit, where the separation circuit has capacitors respectively connected to the first filter and the second filter in series and inductors respectively connected to the first filter and the second filter in parallel, are known (see Patent Document 3: Japanese Patent No. 3,204,753).
Separators as described in the above cited Patent Documents 1 and 2 have a functional feature of diplexer by forming an LC circuit element in a multilayer structure. Basically, both of them are provided with a feature of a filter and a feature of matching the high-frequency side with the low-frequency side by combining the function of a low pass filter and that of a high pass filter. Such filters are formed by using an inductor and a capacitor that are lumped constant elements and hence referred to as lumped constant type filters.
While lumped constant type filters have an advantage that it is not necessary to additionally incorporate a special matching circuit and a relatively high degree of freedom is provided for setting the attenuation pole position because the attenuation pole is produced by an LC resonator, they are accompanied by a disadvantage that resonance peaks appear to reduce the impedance of each lumped constant element to nil in the high-frequency domain where the element gives rise to self resonance to consequently deteriorate the attenuation. Thus, it is necessary to add an attenuation circuit in order to enhance the attenuation characteristic, ending up in an increased number of necessary circuit elements, which by turn obstructs the attempt to downsize the product. Additionally, the number of self resonance peaks increases as the number of circuit elements rises to further deteriorate the attenuation characteristic in the higher harmonic domain.
Furthermore, from the material point of view, materials showing a relatively low dielectric constant will have to be selected to meet the requirement of forming an ideal inductor. A technique of laying a low dielectric constant material and a high dielectric constant material one on the other to form a laminate has been publicized recently so that an inductor may be formed in a region formed by the low dielectric constant material while a capacitor may be formed in a region formed by the high dielectric constant material for the purpose of achieving improved electric characteristics.
Meanwhile, according to Patent Document 3, a filter is formed by using a λ/4 line having a short-circuited side and an open side as resonator and the functional feature of a diplexer is realized by combining two filters and a separator circuit (matching circuit) A filter of the type formed by using a λ/4 line is referred to as distributed constant type filter because it exploits the characteristics attributable to the profiles of the elements.
While a distributed constant type filter generally can achieve higher electric characteristics particularly in terms of attenuation with an arrangement simpler than a lumped constant type filter, it requires a separator circuit (matching circuit) because a diplexer cannot be realized simply by connecting two filters.
Additionally, from the material point of view, a material having a higher dielectric constant is preferably employed for the purpose of downsizing because the resonance frequency is determined by the length of the λ/4 line and the dielectric constant of the material.
As described above, while combinations of lumped constant type elements and those of distributed constant type elements have been proposed to date, no proposal has ever been made to combine a lumped constant type element and a distributed constant type element. The reason for this may apparently be that it is necessary to form a very large resonance element (λ/4 line) in order to reduce the resonance frequency by means of the distributed constant type when a material showing a low dielectric constant is employed, whereas the self resonance frequency of the inductor of the lumped constant type becomes low to consequently narrow the frequency domain necessary for operating as inductor and the attenuation characteristic of higher harmonics is degraded by the self resonance peaks when a material showing a high dielectric constant is employed.
When combining lumped constant type filters and distributed constant type filters, using the technique of laying different materials, combining a high dielectric constant material and a low dielectric constant material, both the lumped constant type filters and the distributed constant type filters need to be laid to form a laminate so that the thicknesses of sheet allocated to the respective types are relatively reduced to give rise to a problem of a reduced distance separating the ground and the inductor particularly of the lumped constant type to consequently degrade the insertion loss.